1. Field of the Invention
The present invention relates to a display apparatus, and more particularly to a display apparatus effecting display operation with a clock signal synchronized in phased with the input video signal.
2. Related Background Art
The so-called CRT display apparatus of raster scan type is being widely used as the display device for the host computers such as work stations (WS) and for the personal computers (PC), and the flat panel display apparatus such as the liquid crystal display panel and the plasma display are recently attracting attention, in consideration of the space saving, energy saving and ergonomics.
Between such host computer and the CRT display apparatus there is exchanged the video signal which is a combination of analog image data and vertical and horizontal synchronization signals or a composite synchronization signal containing such synchronization signals, but such video signal is available in numerous specifications, and the personal computer generally deals with plural video signals of different resolutions.
For example the IBM's PC compatible machine is capable of displaying the images of various video signals for example of 320 pixels.times.200 lines, 640.times.400, 720.times.400, 640.times.350, 640.times.480, 800.times.600, 1024.times.768, 1280.times.1024 etc.
On the other hand, in the field of the CRT display apparatus, there is already known so-called multi-sync CRT display apparatus which detects the state of the synchronization signals of the input video signal and displays the image corresponding to the respective video signal by matching the driving period and the width of the scanning lines with the synchronization signals of the video signal.
In such apparatus it is customary to measure the state of the synchronization signals in advance for certain video signals and to store the result of such measurement as the display parameters in a memory provided in the apparatus. Then, at the detection of the state of the synchronization signals of the input video signal, if the input video signal can be specified by the result of such detection, the display parameters stored in the memory are utilized for achieving satisfactory display.
In contrast to such CRT display apparatus, the dot matrix display apparatus such as the liquid crystal display panel or the plasma display panel is suitable for display control with digital signals, so that the display is usually executed after the input analog image signal is subjected to analog-to-digital conversion.
In such display operation, since the dot matrix display is significantly more difficult to control than the CRT, it is customary to effect the display, in the horizontal direction, by sampling a pixel of the video signal corresponding to each pixel of the display panel. Consequently such display panel is mostly employed in an equipment in which the resolution of the video signal is equal to that of the display panel.
More specifically, though the equipment capable of displaying the images of certain resolution is available, such equipment is combined with a display device and the image display is available only in such combination. Also, even if the image display is available in certain resolving powers, it is still not possible to display the images of widely different resolutions as mentioned above, because the dot matrix display is limited to the simple functions of providing display with a resolving power of the displayed data the same as that of the sampled image data, or with the skipping of the image data with a predetermined interval.
In such dot matrix display apparatus, in order to achieve the functions comparable to those of the multi-sync CRT display apparatus, it is necessary to specify the input video signal and to effect the control matching the resolution.
However, the video signal currently supplied to the CRT display apparatus does not contain the information on the dot clock frequency of the pixels in the horizontal direction, necessary for determining the optimum sampling frequency for A/D conversion.
Also the synchronization signals in the above-mentioned video signals can be widely different in the frequency and the wave form outside the effective image display period. In the conventional multi-sync CRT display apparatus, such differences are not critical, and the measurement of the synchronization signals may be limited to the simple measurement of the frequencies of the horizontal and vertical synchronization signals only in a part of the image display period.
On the other hand, the dot matrix display, relying on A/D conversion with a dot clock signal which is phase synchronized with the input synchronization signal, utilizes a so-called PLL circuit for generating such dot clock signal. Consequently, the above-mentioned variations in the input synchronization signals lead to an aberration in the phase synchronization or an increase in the jitter, thus detrimentally affecting the clock signal generation and eventually resulting in deterioration in the displayed image or instability in the control of the display apparatus.